1. Field of the Invention
The present invention relates generally to control systems for automatic transmissions employed in vehicles, and more particularly, to a lock-up control system for an automatic transmission having a lock-up clutch which is provided between the input and output shafts of a fluid torque converter driven by an engine and operative to couple the output shaft of the fluid torque converter with the output shaft of the engine on the selected occasion in accordance with, for example, engine load and vehicle speed.
2. Description of the Prior Art
There has been commonly used in vehicles an automatic transmission comprising a fluid torque converter and a multi-stage power transmitting gear arrangement including a planetary gear mechanism. In such an automatic transmission equipped with the fluid torque converter and employed in a vehicle, a transmitting gear ratio shifting control, such as shifting up or down control, is usually carried out through an oil hydraulic control circuit provided therein. More specifically, for achieving a required shift of transmitting gear ratio, paths of the oil hydraulic control circuit are switched over therebetween by mechanical or electromagnetic valves to cause friction elements, such as brakes and clutches associated with the multi-stage power transmitting gear arrangement, to operate properly so as to switch over power transmitting paths from one to another in the multi-stage power transmitting gear arrangement. In the case where the paths of the oil hydraulic control circuit are switched over therebetween by the electromagnetic valves to effect the shift of transmitting gear ratio, it is detected that the travelling state of the vehicle has exceeded a predetermined gear ratio range by a certain electronic detector and the electromagnetic valves are caused to operate selectively by the output of the detector.
In the automatic transmission equipped with the fluid torque converter as described above, since a power is transmitted through a fluid in the fluid torque converter, the vehicle having such an automatic transmission employed therein can travel smoothly but, on the contrary, may be accompanied by loss of energy caused due to the slipping of the fluid and resulting in increased fuel consumption.
Accordingly, there has been proposed an automatic transmission having a lock-up clutch which is provided in a fluid torque converter and operative to couple the output shaft of the fluid torque converter directly with the output shaft of an engine, when each of the vehicle speed and engine load becomes a predetermined value or more. In such an automatic transmission equipped with the fluid torque converter having the lock-up clutch in the above manner, however, a lock-up control is required in addition to the aforementioned transmitting gear ratio shifting control, that is, shifting up or down control. In the lock-up control, it has been proposed to release temporarily a lock-up state held by the lock-up clutch during the transmitting gear ratio shifting operation, even if the lock-up clutch is situated to be to operate, for the purpose of avoiding the shock of great degree which may arise from the shift of transmitting gear ratio carried out under the lock-up state held by the lock-up clutch left in operation.
However, when the lock-up state is released in accelerating the vehicle with an accelerator pedal stepped on with a driver's foot, the speed of the engine is rapidly increased due to the slip of the fluid in the fluid torque converter. This is out of a problem in the shifting down control because the speed of the engine is required to be increased by the amount corresponding to the increased gear ratio. In the shifting up control by which the speed of the engine is to be lowered, if the timing of releasing the lock-up state is too early, there is raised such a problem that the speed of the engine is first increased transitorily too much so as to induce a so-called spout up of the engine, then lowered, and this results in unpleasantness to a driver.
For avoiding such a problem, for example, as disclosed in U.S. Pat. No. 4,431,095, issued to M. Suga, on Feb. 14, 1984, there has been proposed a method of control in which a lock-up releasing signal for causing a lock-up clutch to be in its inoperative state is delayed by a predetermined constant period of time in relation to a shifting up signal, which is supplied to cause the multi-stage power transmitting gear arrangement to effect the shifting up operation therein, to be supplied to a corresponding oil hydraulic actuator provided in an oil hydraulic control circuit for controlling the lock-up clutch and the multi-stage power transmitting gear arrangement.
Generally, in oil hydraulic control circuits for a vehicle, the pressure (line pressure) of an oil to be fed into oil hydraulic actuators is controlled to be heightened when a throttle valve of an engine is made open, and to be lowered when the throttle valve is closed. The reason for this is that, since the output torque of the engine is increased with the throttle valve opened, it is required to increase the line pressure acting on the oil hydraulic actuators for causing clutches and brakes to operate so as to bring about increment in power transmitting capacity of the clutches and in braking force of the brakes. Since the operating period of time of each oil hydraulic actuator in the automatic transmission is shortened when the line pressure is thus increased, the period of time of each control for shifting the transmitting gear ratio is also generally shortened. On the other hand, because the pressure in the fluid torque converter is kept substantially constant by a requlator or check valve independently of the line pressure from the viewpoint of a transmitting efficiency, the period of time required for releasing the lock-up state held by the lock-up clutch can not vary depending on the level of the line pressure.
Consequently, in the case where the delay time of the lock-up releasing signal is kept constant on the ocassion of the shifting up operation in the multi-stage power transmitting gear arrangement, as described above, the problems that the shock of great degree is caused by the shift of transmitting gear ratio with a certain opening degree of the throttle valve and further that the spurt up of the engine is induced by the shift of transmitting gear ratio with another opening degree of the throttle valve, may arise.